The present invention relates to a brake circuit apparatus for a hydraulic motor, and more particularly to a brake circuit apparatus for a hydraulic motor in a hydraulic circuit system of the construction machine such as a hydraulic excavator in which there is provided a mechanical brake device including a brake release cylinder adapted to release the rotary shaft of a hydraulic motor from the braking by the mechanical brake device by supply of hydraulic fluid to the brake release cylinder.
The brake circuit apparatus for a hydraulic motor has hitherto been known for example in a hydraulic circuit system for use in a hydraulic excavator. As shown in FIG. 1, the conventional brake circuit apparatus comprises main hydraulic pumps 1, 2, a swing hydraulic motor 3, for swing, hydraulic motors 4, 5 for left and right travels, hydraulic cylinders for front attachments, i.e., a boom cylinder 6, an arm cylinder 7 and a bucket cylinder 8, which are driven by hydraulic fluid supplied from the main pumps 1, 2, and directional control valves for these respective motors and cylinders for control of the flow rate and flow direction of hydraulic fluid supplied to the motors and cylinders from the main pumps, i.e., a directional control valve 9 for swing, directional control valves 10, 11 for left and right travels, first and second directional control valves 12a, 12b for boom, first and second directional control valves 13a, 13b for arm, and a directional control valve 14 for bucket, the swing hydraulic motor 3 being provided with a mechanical brake device 15 for braking its rotary shaft 3a. A provisional directional control valve 16 is also incorporated, in the circuit system. The main hydraulic pump 1 has its fluid supply line 1a connected through a center-bypass line 1b passing through the directional control valves 9, 13a, 12a, 16 and 10 to inlet ports 9a of those control valves (the inlet ports and their connections to the line 1b for those control valves other than the valve 9 are not shown for purposes of brevity), while the outlet ports 9b of these control valves are connected to a return line 1c (likewise, the outlet ports and their connections to the line 1b for those control valves other than the valve 9 are not shown), the center-bypass line 1b and return line 1c both being connected to a reservoir 17. Although not shown, the other directional control valves 11, 14, 12b and 13b are also connected to the main pump 2 and reservoir 17, in like manner. Between the directional control valve 9 and the swing motor 3 is interposed a relief valve unit 23, and it is to be noted that between the other control valves and their associated motors or cylinders are also provided similar respective relief valve units, not shown.
The swing control valve 9 is actuated by an operation device 18 which includes a control level 18a and two pilot valves 18e and 18f connected to a fluid supply line 18c extending from a pilot pump 18b and to a return line 18d leading to the reservoir 17, respectively, and adepted to be selectively actuated by the control lever 18a. Those pilot valves 18e and 18f are further connected to pilot-operated sections 9c and 9d of the swing valve 9 through pilot lines 18g, 18h, respectively. Although not shown, the other control valves 10, 11, 12a and 12b, 13a, 13b are likewise associated with similar operation devices.
The mechanical brake device 15 includes a brake release cylinder 19 for releasing the rotary shaft 3a of the swing hydraulic motor 3 from the braking thereby by supply of hydraulic fluid to the brake release cylinder, and the brake release cylinder 19 is connected through a line 19d to an outlet 19c of a shuttle valve 19f provided in a line 19e connected between the pilot lines 18g and 18h, thereby providing a brake circuit apparatus 20. In the line 19d, a check valve 21 and a restrictor 22 are connected in parallel with each other, the check valve 21 allowing fluid flow to be directed toward the brake release cylinder but prohibitting the same in the reverse direction.
The rotary shaft 3a of the swing motor 3 carries a driving gear 3c mounted through a speed reducer 3b in a known manner, the driving gear 3c being in meshing engagement with an internal gear of a swing frame, not shown.
In the above-mentioned hydraulic circuit system, the brake circuit apparatus 20 for the swing motor 3 operates as follows. When the swing control valve 9 is in its neutral position with the operation device 18 being inactivated, the fluid from the pump 1 flows downstream of the valve 9 through the fluid supply line 1a and center-bypass line 1b, and further, depending on the operating conditions assumed by the downstream control valves 13a, 12c and 10, it flows either directly to the reservoir or to the reservoir 17 through the return line 1c after having actuated the cylinder or motor associated with the operated valve.
Assuming here that the control lever 18a of the operation device 18 is actuated in a direction shown at W in FIG. 1, a pilot pressure from the pilot pump 18b is transmitted to the line 18g through the pilot valve 18e, and it is further transmitted through the line 19e, shuttle valve 19f and its outlet 19c, line 19d, and check valve 21 to the cylinder chamber 19a of the brake release cylinder 19. Thus, against a force of the spring 19b, the pilot pressure acts to release the braking of the rotating shaft 3a of the motor 3, thereby rendering the motor 3 freely rotatable. Meanwhile, the pilot pressure transmitted through the pilot line 18g is applied to the pilot-operated section 9c of the directional control valve 9, so that the valve is shifted from neutral to the left side working position. This permits the hydraulic fluid discharged from the pump 1 to flow into the valve 9 through the supply line 1a, center-bypass line 1b and inlet port 9a, and then into the motor 3 while a return fluid from the motor 3 flows back into the valve 9 and then into the reservoir 17 through the outlet port 9b and return line 1c, thereby causing the hydraulic motor 3 to be activated.
When the control lever 18a is restored to its neutral position, the pilot valve 18e is brought into communication with the reservoir 17, thereby the pilot-operated section 9c of the control valve 9 as well as the pilot line 18g also being brought into communication with the reservoir 17. This causes the valve 9 to be shifted back to its neutral position. Instantaneously, the pressure in the line 19d is caused to reduce, and the pressure in the chamber 19a of the brake release cylinder 19 is also caused to reduce, so that the mechanical brake device 15 is actuated so as to brake the rotary shaft 3a of the motor 3. In this instance, however, the fluid within the cylinder chamber 19a is forced to flow through the restrictor 22 since it is not permitted to flow through the check valve 21. This implies that the pressure reduction within the chamber 19a takes time. During this period, the directional control valve 9 closes its outlet port 9b, and the hydraulic motor 3 stops its rotation as a result of being hydraulically braked. Then the rotary shaft 3a of the motor 3 is locked or breaked by the mechanical brake device 15.
In the brake circuit apparatus as described above, when the operation device 18 is actuated to shift the directional control valve 9 for actuation of the motor 3, there involves the necessity of initiating rotation of the motor 3 after having released the braking by the mechanical brake device 15, since transmission of the torque of the motor 3 to the mechanical brake device 15 may result in reduction of serviceable life of the mechanical brake device.
According to the conventional brake circuit apparatus, however, the pilot pressure as an output of the pilot valve 18e or 18f is commonly used for operation of the brake release cylinder 19 and for shifting of the directional control valve 9 while the shuttle valve 19b is additionally provided in the line leading to the release cylinder 19, and this arises a time lag between the establishment of pilot pressure and the actual release of the braking by the mechanical brake device by supply of a fluid flow rate necessary for actuation of the brake release cylinder 19 after the established pilot pressure has been transmitted to the cylinder 19. Further, the pilot pressure being established is proportional in magnitude to the extent of stroke of the actuated operation device 18, and therefore when the extent of stroke of the operation device is small, the pilot pressure as established is also small correspondingly, and this has resulted in furtherance of the above-described time lag. Thus, the problem has been frequently encountered in that the motor 3 initiates rotation before the braking by the mechanical brake device 15 is fully released.
In the conventional hydraulic brake apparatus for the swing hydraulic motor, as described so far, when the operation device 18 for the swing motor 3 is actuated, the rotary shaft 3a of the motor 3 is released from the braking by the mechanical brake device, enabling rotation of the motor 3. However, since the operation devices, not shown, associated with the other cylinders and motors 4 to 8 are not associated with the brake circuit apparatus 20 at all. The braking of the shaft 3a of the motor 3 is not released even if those operation devices are actuated. This may cause the following inconvenience. The operations performed by the hydraulic excavator involve an excavating operation in which only operations of the front attachments, i.e., boom, arm and bucket are required. During such excavating operation, it may happen that the side faces of the bucket is turusted by soil being excavated or the hydraulic excavator is operated particularly in a sloping terrain, so that the swing frame of the excavator is objectionably acted upon by an swing force due to the thrusting or the rotational moment caused by the weights of the front attachments. Under such circumstances, the result is that since the braking of the rotary shaft of the swing motor is not released, the swing force must be taken up or borne by both a relief torque caused by the relief valve unit 23 for the swing motor 3 and the braking force of the mechanical brake device 15. This causes an excessive load applied to the speed reducer 3b disposed between the driving gear 3c in meshing engagement with the internal gear of the swing frame and the mechanical brake device 15, which may possibly reduce its durability.
Accordingly, an object of the present invention is to provide a hydraulic brake apparatus for a hydraulic motor which is capable of effecting positive release of the braking by a mechanical brake device before the hydraulic motor starts its rotation.
A further object of the invention is to provide a hydraulic brake apparatus for a swing hydraylic motor in a hydraylic circuit system of the construction machine which is capable of preventing an excessive load from being applied to the speed reducer for the swing hydraulic motor in the operation in which only operations of the front attachments are required and the operation of the swing frame is not required, so that the durability is not reduced.